Prescription drug therapy is widely used to treat various medical conditions and disorders. Many prescription drugs produce side effects and adverse reactions in subjects, which can lead to considerable discomfort and poor quality of life. Also, many prescription drugs interact with other drugs and/or with foods such that they do not work as intended when taken alongside these other drugs/foods. Drug-drug and/or drug-food interactions can mean that prescription drugs with a narrow therapeutic window (such as warfarin, lidocaine, digoxin, lithium, phenytoin, etc.) may become toxic, producing many severe side effects and strong adverse reactions even at low dosing levels.
A system for mitigating this issue for use by medical professionals is described in US 2002/0095313. The US '313 system requires all prescriptions and drug orders to be generated using a computer having a built-in error correction system. The error-correction system has access to data on which drugs have already been prescribed for a given subject and/or foods the subject eats, as well as a database containing information on drug-drug interactions and drug-food interactions. When a new drug is prescribed for the subject, the system uses the database to check for interactions between the new drugs and the already prescribed drugs and/or the foods the subject eats and produces an alarm if an interaction is found.
An alternative system, for use by subjects, is described in EP 1363225. The EP '225 system provides a user with a barcoded subscriber card, where the barcode uniquely identifies the user, and a device having a barcode reader. The user scans the barcode of a product (e.g. a food product or an over-the-counter medication) and their subscriber card, and the device communicates the barcode information to an application that looks up (e.g. by using an internet connection to access one or more central databases) the user's profile (which contains information about the user's prescriptions, allergies, chronic illnesses and/or other user specific information product ingredients) and the ingredients of the product. The application compares the ingredients to the user's profile and the results of the comparison are sent back to the user.
Both of these systems suffer from the drawback that the food and drug interactions which may be experienced by a given subject are identified solely based on a database of known drug and food interactions. They cannot detect interactions which are not documented in the databases, and they cannot detect or monitor side effects and/or adverse reactions which the subject actually experiences. However; the huge variety of different prescription drugs, over-the-counter drugs and foods which are available means that many, if not most, of the possible combinations of a given drug with another drug or food have not been tested for adverse reactions. Furthermore, it may often be the case that a drug is taken alongside more than one other drug and/or food that could interact with it. In such situations the interaction is unlikely to have been documented, and the resulting effects can be difficult to predict from first principles.
It is therefore desirable to be able to detect the side effects and adverse reactions due to drug-drug and/or drug-food interactions which are actually experienced by a subject taking a given drug. By knowing the actual side effects and adverse reactions experienced by a subject, one can take measures to relieve them, for example by varying the drug dose prescribed. In this way, the health and quality of life of a subject can be improved considerably. Furthermore, it is desirable to detect such side effects and adverse reactions at an early stage so that a severe adverse reaction can be avoided.
Examples of circumstances in which a subject, caregiver or physician may want to be able to detect and/or monitor the side effects and adverse reactions that are experienced by a subject taking a particular drug include:                if the subject wants to lead an active and normal daily life but is unable to do so because of the discomfort or malaise caused by severe side effects and adverse reactions;        if the caregiver or physician wants to determine whether the drug dose needs to be altered during treatment for a particular medical condition to improve therapeutic effectiveness or prevent toxicity;        if the subject wants to begin taking a particular new drug or food alongside their existing drug therapy;        if the physician or caregiver wants to closely manage the therapy of a high risk subject, e.g., one who has liver or kidney failure;        if the physician wants to determine the best drug dose for a specific subject to minimizes side effects and adverse reactions;        to find out whether there has been an error in the drug dose prescribed;        to determine the best time for administering a drug;        if the physician wants to closely control the administration of a drug with a narrow therapeutic window which may become toxic even at low dosing levels.        
There is therefore a need for an improved method and device that can provide a reliable determination of whether a given subject is experiencing a side effect or adverse reaction to a drug they are taking. Such a method and device could be used in a home or hospital-based monitoring system to detect the occurrence of side effects and adverse reactions, as well as for monitoring the progression of such side effects and adverse reactions as a result of adjusting the drug therapy.